1. Field of the Invention
This invention relates to a piezoelectric pick-up device for an internal combustion engine, which is mounted on an internal combustion engine cylinder wall or the like for electrically detecting the knocking or the like generated in the internal combustion engine with a piezoelectric element.
2. Description of the Prior Art
A piezoelectric pick-up device of non-resonance type is well known in the art, in which a piezoelectric element and a weight thereon are disposed in and on the inner bottom surface of a metal case secured to an internal combustion engine cylinder wall or the like with the piezoelectric element electrically coupled to the metal case. When a knocking or like vibrations are generated, the piezoelectric element generates an output signal corresponding to a strain produced in the cylinder wall due to vibrations of the weight caused by gravitational acceleration thereof accompanying the knocking or the like. The knocking or the like is thus detected by the device.
In such a prior art piezoelectric pick-up device for an internal combustion engine, as shown in FIGS. 1 and 2, the interior space b of a case a, in which a piezoelectric element c and a weight d are disposed, is filled with a rubber-like molding material e, e.g., soft epoxy resins, to directly cover the piezoelectric element and the weight in order to suppress spurious vibrations and also provide an airtight enclosure. Changes in the elasticity of the rubber-like molding material with changing temperature, particularly hardening of the molding material due to a drop in temperature, would have serious adverse effects on the sensor characteristics.
In case of a sealed structure as shown in FIG. 2, where the open top of the interior space b is sealedly closed by a lid f, volume changes of the molding material e are not allowed. Therefore, an expansion force produced in the molding resin with a temperature rise would apply an excess load on the piezoelectric element c and the weight d to adversely affect the output of the pick-up device. In either structure of FIG. 1 or 2, the temperature characteristics have been inferior.